<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .d1{
            width: 100px;
            height: 100px;
            background-color:red;
            float: left;
            /* 并排效果 */
        }
        .d2{
            width: 100px;
            height: 100px;
            background-color:blue;
            float: left;
        }
        .d3{
            width: 120px;
            height: 120px;
            background-color:green;
            /* d3认为d1，d2在空中，他自己在地上  俯视效果就是被覆盖 */
            /* 解决方案：告诉d3，  d1，d2没有在空中 用clear：left*/
            clear: left;
        }
    </style>
</head>
<body>
    <div class="d1">1</div>
    <div class="d2">2</div>
    <div class="d3">3</div>
</body>
</html>